A dual-fuel internal combustion engine is an engine which can operate as a full diesel cycle engine in which combustion is by compression ignition, or as an Otto cycle natural gas engine in which combustion is started through the compression ignition of a small quantity of liquid diesel fuel injected into the compressed gas-air mixture. Therefore, a dual-fuel engine is a gas-fuel burning engine in which the gaseous fuel-air mixture is ignited through the compression ignition of liquid diesel fuel instead of ignition with spark plugs as in a spark ignited gas fueled engine. As a result, the dual fuel engine must be capable of operating in two entirely different modes.
In the diesel fuel mode of operation, combustion occurs in an "in situ" process, where each droplet of diesel fuel is a potential ignition source. It is essential to provide the injected fuel with sufficient oxygen to enable all the fuel droplets to fully ignite to achieve maximum combustion efficiency. A deficiency of oxygen will lead to incomplete combustion, resulting in a loss of combustion efficiency, higher thermal loads, and excessive exhaust smoke. For this reason, it is preferable to supply a diesel engine with as much air as is practicable in order to maximize operating efficiency and combustion stability. For all practical purposes, and with engine structure limitations, the diesel power cycle has no upper limit on the amount of air used for combustion. Therefore, once minimum levels have been established, the term air/fuel ratio for a diesel engine is essentially inappropriate and has little meaning.
A natural gas burning engine, on the other hand, has very different requirements. In the gas burning process, combustion is completed through flame front progression across the face of the combustion chamber. This process relies almost entirely on the interfacing of hot burning flame front gases igniting the yet unburned portion of the gas-air mixture. Thus, if the flame front progression across the combustion chamber were interrupted for any reason, the balance of the fuel-air mixture would remain unaffected, and an incomplete Combustion cycle would occur.
A partially incomplete combustion cycle is nowhere as dangerous as a complete combustion failure, commonly termed a "misfire." If misfire occurs with sufficient frequency and in a sufficient number of power cylinders, an accumulation of unburned gas-fuel mixture may "load up" in the exhaust system. If the unburned gas-fuel mixture were inadvertently ignited, an exhaust explosion would occur which could have sufficient force to split the exhaust muffler from top to bottom. Since the nature of combustion in a gas-fired engine is so heavily dependent on the air-to-fuel ratio, complete combustion failure can occur from an excess supply of combustion air or "lean misfire," to combustion failure due to a deficiency of combustion air or "rich misfire." Both lean and rich misfire conditions are undesirable and unsafe. An even more threatening problem to the engine structure is "detonation," in which a supersonic combustion shock wave is created by auto ignition of the unburned portion of the gas charge. Unchecked severe detonation is capable of destroying an engine.
Accordingly, unlike the full diesel engine, combustion air control in a natural gas internal combustion engine is critical for the success and safety of engine operation. In a dual-fuel internal combustion engine, it becomes imperative that maximum, unrestricted combustion air supply is available when the engine is operating in the full diesel mode, and that combustion air supply be modulated when operating in the dual-fuel or gas fuel mode.
Conventional combustion air control systems which regulate the air/fuel ratio have been available. Through the years, many approaches have been used to devise systems suitable to modulate the air supplied for gas combustion. An almost universal approach in the conventional systems is to gauge the flow of fuel gas being supplied to the engine, and then modulate the flow of combustion air to achieve the target air-fuel conditions.
There are several definitions of the term "air/fuel ratio." Some of the definitions are listed in the following table:
______________________________________ Weight basis Air/Fuel Ratio LBW/LBW Weight Flow basis Air/Fuel Ratio (LBW/HR)/(LBW/HR) Volume basis Air/Fuel Ratio FT.sup.3 /FT.sup.3 Actual Volume Flow basis Air/Fuel ACFH/ACFH Ratio Standard Vol. Flow basis Air/Fuel SCFH/SCFH Ratio Mass Flow basis Air/Fuel Ratio (LBM/HR)/(LBM/HR) Total Air/Total Fuel Ratio VOLUME, MASS Stoichiometric Air/Fuel Ratio VOL/VOL, LBM/LBM Excess Air/Fuel Ratio VOL/VOL, LBM/LBM Gage Pressure basis Air/Fuel Ratio PSIG/PSIG Absolute Pressure basis Air/Fuel PSIA/PSIA Ratio Trapped Air/Fuel Ratio VOL/VOL, LBM/LBM ______________________________________
In order to discuss air/fuel ratio with some degree of confidence one needs to know the specific basis for the discussion. Because of the many forms of "air/fuel ratio," a casual discussion or decision can have serious downside effects. For instance, the weight basis air/fuel ratio for Methane (CH.sub.4) is 17.22 pounds of air for each pound of Methane. However, on a volume basis, one cubic foot (FT.sup.3) of Methane requires 9.54 FT.sup.3 of air. The difference is found in the molecular weight differences between methane and air.
To further complicate the discussion, the stoichiometric air/fuel ratio, also known as the chemically correct air/fuel ratio, cannot be run on a turbo charged engine because of the high overlap scavenging required for combustion chamber cooling. An attempt to use the stoichiometric air/fuel ratio on a turbo charged engine may cause thermal destruction of the engine.
Other less obvious but equally troublesome air/fuel ratio applications include the pressure basis air/fuel ratio. With this concept, the gas fuel header pressure is measured and the manifold air pressure is balanced against the fuel gas pressure. While this is a common method of controlling a gas engine, some operating conditions may severely and irreversibly upset this air/fuel balance. The conventional force balance opposing diaphragm cross link-connected arrangement is one example of this system. Some inherent problems with the pressure basis air/fuel ratio includes the temperature of the gas and air media. In other words, if the control device is set up for a given set of conditions, changing those conditions will change the response characteristic of the controller.
An analogy of this problem is a gallon bucket filled to the brim with water at 60.degree. F. The water line will be exactly even with the bucket top rim. Now raise the water temperature by heating the bucket with a torch to a temperature of 100.degree. F. The thermal expansion of the water will cause some of the water to spill over the bucket sides When cooled back to 60.degree. F., the water that spilled onto the floor will not return to the bucket but will remain on the floor. The bucket now contains less water and the water line will be substantially below the bucket rim.
The same situation takes place with a gas, but the effect is amplified because heat energy (BTU) is involved. If an engine is set up to run satisfactorily with fuel gas at, for example, 60.degree. F., the horsepower flowing to the engine in the form of fuel gas energy will be sufficient to balance the horsepower required by the driven equipment and heat rejection loads. Under these conditions one cubic foot of fuel gas will contain a certain heat content or heating value expressed as BTU/FT.sup.3.
Heating the fuel gas to 100.degree. F., for example, causes some of the fuel gas to "spill out" of the standard cubic foot. If the fuel gas were cooled back to 60.degree. F., by the water bucket analogy, there would be less gas in the standard cubic foot. Therefore, although a cubic foot would still measure a cubic foot at the higher fuel gas temperature there would actually be 7% fewer BTUs available. Because of the higher fuel gas temperature and lower heat energy per cubic foot, there would not be sufficient energy input to the engine to satisfy the load and heat rejection requirements. As a result the engine will lose output power evidenced by a drop in RPM, torque or both. The only way to get more heat energy to the engine with the higher gas temperature is to "pack" more gas into the already "full" cubic foot. This additional "packing" will raise the pressure inside the cubic foot. In an actual engine, the result will be an increase in the fuel gas header pressure only to regain the energy lost due to fuel gas heating. No additional output horsepower will be experienced. The specific heat input to the engine, BTU/HP-HR will be essentially constant, the only difference being the increased pressure in the fuel gas header. If the air control device were to ignore the fuel gas pressure, no effect would be seen in engine performance evidenced by exhaust temperatures, power output, etc.
However, an engine air control device using fuel gas pressure as a set point will see the change in fuel gas pressure caused by the change in fuel gas temperature, and will correct the air supplied to the engine based on this apparent change in engine load when, in fact, the engine load did not change. The result may be an engine pulling the same load with detonating cylinders because of lower combustion air supply brought on by cooler gas or a kilowatt meter cycling wildly, because of an excess of combustion air caused by higher fuel gas temperature, in all cases, because the air supplied to the engine is tied to the fuel gas header pressure. Similar conditions would be experienced with a controller using governor position as a set point, because the engine governor controls the fuel gas flow control valve and the fuel gas header pressure. A combustion air control device which senses fuel gas header pressure as a basis for engine air control level is further disproportioned when normal maintenance is practiced on the engine. Adjusting the individual cylinder fuel gas supply will affect the gas header pressure overall, which will therefore affect engine air supply in a manner similar to fuel gas temperature effects.
Controlling a gas fired engine on the basis of air/fuel ratio alone will not produce the results expected. Better success might be expected if the air/fuel ratio at the spark plug or at the fuel injector were sampled at ignition time. Hardware unavailability prevents this type sampling on a routine, cost effective, and continuous basis with current technology. Furthermore, scavenging requirements would not be addressed with this technique.
Even exhaust gas oxygen sampling, although routinely used in the non-supercharged or naturally aspirated engines, is not suitable for the turbo charged engine with high overlap periods. With the scavenging part of the power cycle spanning nearly 20% of the cycle time, free unreacted oxygen levels in the range of 10 to 12% make micro-controlling difficult.
A departure from the classical "measure gas, and then set air" method of modulating combustion air supply involves inferring the optimum air fuel ratio by "looking back" at the engine's exhaust gas outlet temperature. It has been found that when properly proportioned, the optimum or near optimum air/fuel ratio results in a nearly constant engine exhaust outlet temperature, regardless of load. While this approach provides some definite advantages in system simplicity, it also requires near constant "before engine" input conditions. There are some situations where unwanted effects are produced. One example is where a change in combustion air temperature can cause a reverse exhaust temperature effect because of the nature of gas fuel combustion. In other words, for a given combustion air density, defined in terms of air manifold temperature and pressure, the lowering of the air manifold temperature will provide greater air density than the reference condition. Because an increase in air density causes a retardation in the rate of gaseous fuel combustion, the exhaust or combustion discharge temperature will show an increase. This increase in exhaust temperature would cause the control device to supply a greater supply of combustion air to cool the exhaust temperature down, resulting in a further increase in the combustion air density. Therefore, the result in engine air control is opposite from that desired.
In addition to the effects of air manifold temperature on air charge density is the effect of fuel transfer from the diesel mode to the dual-fuel or gas fuel mode. While operating in the diesel mode, all air bypass valves intended to modulate combustion air flow are fully closed in order to provide maximum air to the diesel power cycle. In this condition, the exhaust temperature is at the lowest setting for any given engine load. When the engine is transferred to dual-fuel operation, the conventional exhaust temperature based control system is activated and an exhaust temperature set point is established. This exhaust temperature set point is almost always higher than the diesel exhaust temperature. Because of the sometimes large difference between diesel exhaust temperature and dual-fuel exhaust temperature, the conventional exhaust temperature based air control system acts to restrict air flow to the engine to attempt to raise the exhaust temperature to the dual-fuel set point. This action invariably causes a reduction in air supply, sometimes low enough to drive the gas combustion process into detonation, which if left unchecked could damage the engine.
The diesel to dual-fuel transfer problem cannot be as easily addressed with a biasing approach because of the uncertainty of fuel transfer duration or transfer times. Attempts have been made to lock out control for some time before enabling the controller, but this is still a disadvantageous "feed forward" approach.